3Com 4800G Series, Configuration Manual

Page: 582 / 1246

582 C
HAPTER
43: MLD S
NOOPING
C
ONFIGURATION
Membership reports
A host sends an MLD report to the multicast router in the following circumstances:
■
Upon receiving an MLD query, an IPv6 multicast group member host responds
with an MLD report.
■
When intended to join an IPv6 multicast group, a host sends an MLD report to
the multicast router to announce that it is interested in the multicast
information addressed to that IPv6 multicast group.
Upon receiving an MLD report, the switch forwards it through all the router ports
in the VLAN, resolves the address of the reported IPv6 multicast group, and
performs the following to the receiving port:
■
If no forwarding table entry exists for the reported IPv6 multicast group, the
switch creates an entry, adds the port as member port to the outgoing port list,
and starts a member port aging timer for that port.
■
If a forwarding table entry exists for the reported IPv6 multicast group, but the
port is not included in the outgoing port list for that group, the switch adds the
port as a member port to the outgoing port list, and starts a member port
aging timer for that port.
■
If a forwarding table entry exists for the reported IPv6 multicast group and the
port is included in the outgoing port list, which means that this port is already a
member port, the switch resets the member port aging timer for that port.
n
A switch does not forward an MLD report through a non-router port. The reason
is as follows: Due to the MLD report suppression mechanism, if the switch
forwards a report message through a member port, all the attached hosts listening
to the reported IPv6 multicast address will suppress their own reports upon
hearing this report, and this will prevent the switch from knowing whether any
hosts attached to that port are still active members of the reported IPv6 multicast
group.
Done messages
When a host leaves an IPv6 multicast group, the host sends an MLD done message
to the multicast router.
When the switch receives a group-specific MLD done message on a member port,
it first checks whether a forwarding table entry for that IPv6 multicast group exists,
and, if one exists, whether its outgoing port list contains that port.
■
If the forwarding table entry does not exist or if its outgoing port list does not
contain the port, the switch discards the MLD done message instead of
forwarding it to any port.
■
If the forwarding table entry exists and its outgoing port list contains the port,
the switch forwards the done message to all router ports in the VLAN. Because
the switch does not know whether any other hosts attached to the port are still
listening to that IPv6 multicast group address, the switch does not immediately
removes the port from the outgoing port list of the forwarding table entry for
that group; instead, it resets the member port aging timer for the port.
Upon receiving an MLD done message from a host, the MLD querier resolves from
the message the address of the IPv6 multicast group that the host just left and